docs: def fetch_pdb(self, pdb_code):
          """Fetches PDB file contents from the Protein Databank at rcsb.org.
  
          Parameters
          ----------
          pdb_code : str
              A 4-letter PDB code, e.g., "3eiy".
  
          Returns
          ---------
          self
  
          """
          self.pdb_path, self.pdb_text = self._fetch_pdb(pdb_code)
          self._df = self._construct_df(pdb_lines=self.pdb_text.splitlines(True))
          return self